1. Field of the Invention
The present invention relates to an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu having a high critical current density, and a method for manufacturing the same.
2. Description of the Related Art
An oxide substrate or an oxide board such as MgO, alumina or YSZ, or a metal substrate or a metal board such as Ag, Au, Pt or Ni is formed with an oxide superconductor in film form to function, so as to devise a variety of applied products.
As a method of forming this oxide superconductor into film form, a technique is tried in which oxide superconductive synthetic powder that is oxide superconductor powder, to which an appropriate organic binder is added, is made into paste form, thereafter applied to the surface of the substrate or the board using the screen printing method, the doctor blade method, the spray method, or the like, and burned to thereby form a polycrystalline oxide superconductor thick film.
This technique of forming the oxide superconductor thick film has a quite low manufacturing cost, that is, an advantage because of no need for an expensive single crystal substrate and a large-scale and expensive apparatus requiring a high vacuum system represented by PVD, CVD or the like, and thus it is considered as the technique closest to practical use.
Conventionally, as an oxide superconductor materiel for forming this thick film, it has mainly been tired to use the YBCO family having a composition of Y1Ba2Cu3Ox, the Bi2212 family having a composition of Bi2Sr2Ca1Cu2Oy, and the Bi2223 family having a composition of (Bi, Pb)2+aSr2Ca2Cu3Oz. (Note that, generally, 0 less than a less than 0.5, and this applies to the following.)
When the oxide superconductor thick film is applied here to a practical product, a critical current density (hereafter, described as xe2x80x9cJcxe2x80x9d) of 3,500 A/cm2 or more is required at the temperature of liquid nitrogen.
From this point of view, the conventional art of the above-described oxide superconductor is briefly explained.
For example, an oxide superconductor thick film (hereinafter, described as a xe2x80x9cthick filmxe2x80x9d) containing Y1Ba2Cu3Ox can attain a Jc of several hundred A/cm2 at the temperature of liquid nitrogen. A critical current value (hereafter, described as xe2x80x9cIcxe2x80x9d) of about several A can be attained in a thick film sample thereof having a width of 5 mm and a thickness of 100 xcexcm.
It is difficult, however, to align the crystal orientation of Y1Ba2Cu3Ox in the form of the thick film of the oxide superconductor, that is, a polycrystal, and accordingly it is difficult to attain the aforesaid Jc or more.
Further, for example, a thick film containing Bi2Sr2Ca1Cu2Oy can attain a Jc of 6000 A/cm2 at the temperature of liquid nitrogen, an Ic of about 10 A to about 15 A in a thick film sample thereof having a width of 5 mm, a thickness of 50 xcexcm and so on, and thus it is a potential material.
In this material, however, superconducting characteristics sharply decrease as the film thickness of the sample is increased to attain a higher Ic. As a result, with an increase in film thickness, the value of Jc, obtained by averaging the values of the whole thick film, decreases, such that even a thick film produced having, for example, a width of 5 mm and a thickness of 100 xcexcm has an Ic of about 20 A and a Jc of as small as about 4000 A/cm2. Even if the film thickness is increased more, the Ic does not increase in proportion thereto, but conversely the averaged Jc keeps on decreasing.
Further, for example, as for a thick film containing (Bi, Pb)2+aSr2Ca2Cu3Oz, a thick film sample having a thickness of 100 xcexcm is currently manufactured and can attain a Jc of several hundred A/cm2 at the temperature of liquid nitrogen, and an Ic of about several A in a thick film sample thereof having a width of 5 mm. The Jc exhibited by this thick film, however, is about one-tenth of the value expected from the value of a Jc offered by a similar bulk body containing (Bi, Pb)2+aSr2Ca2Cu3Oz. Furthermore, when the film thickness of the thick film is increased to be 100 xcexcm or more to attain a higher Ic, the oxide superconductor peels from the surface of the substrate or the board in the aforesaid burning, and thus the oxide superconductor containing (Bi, Pb)2+aSr2Ca2Cu3Oz is considered to be difficult to use as a thick film material.
Under the above-described situation, a problem that the present invention solves is to provide an oxide superconductor thick film having a high Jc and Ic and a method for manufacturing the same.
A first means for solving the above problem according to the present invention is an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu to be formed on a surface of a substrate or a board, characterized in that
a film thickness of the thick film is 130 xcexcm or more.
By setting the film thickness of the oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu to be formed on a surface of a substrate or a board at 130 xcexcm or more, the Jc and Ic exhibited by the thick film can be greatly improved.
A second means is an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu to be formed by applying oxide superconductive synthetic powder onto a surface of a substrate or a board and burning the powder, wherein
30 wt % or more of Pb contained in the applied oxide superconductive synthetic powder is retained in the thick film.
The oxide superconductor thick film which has the above-described configuration and contains Bi, Pb, Sr, Ca and Cu to be formed on the surface of a substrate or a board has a high Jc and Ic.
A third means is an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu to be formed on a surface of a substrate or a board, characterized in that
where oxide superconductive synthetic powder having a mole ratio of Bi:Pb:Sr:Ca:Cu=1.85:0.35:1.90:2.05:3.05 is burned at 850xc2x0 C. for 100 hours and then compressed with a pressure of 3 ton/cm2 using a CIP, and further burned at 850xc2x0 C. for 100 hours to prepare a bulk body of an oxide superconductor containing (Bi, Pb)2+aSr2Ca2Cu3Oz (Note: 0 less than a less than 0.5), and a strength of a signal of Pb contained in a center part of the bulk body is measured by an EPMA and the signal strength is normalized as 1,
when a strength of a signal of Pb contained in the thick film direct on the surface of the substrate or the board is measured by the EPMA, a relative strength of the signal of Pb is 0.67 or more.
The oxide superconductor thick film which has the above-described configuration and contains Bi, Pb, Sr, Ca and Cu to be formed on the surface of a substrate or a board has a high Jc and Ic.
A fourth means is an oxide superconductor structure, characterized in that the oxide superconductor thick film according to any one of the first to the third means is formed on a surface of a substrate or a board.
According to the above-described configuration, an oxide superconductor structure having a high Jc and Ic can be obtained at a low manufacturing cost.
A fifth means is a method for manufacturing an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu to be formed on a surface of a substrate or a board characterized by comprising the steps of:
when producing the thick film, applying an oxide superconductor paste to the surface of the substrate or the board to have a film thickness of 260 xcexcm or more, and burning the paste at burning temperatures of 830xc2x0 C. to 850xc2x0 C.
By employing the above-described manufacturing method, it is possible to manufacture an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu and having a film thickness of 130 xcexcm or more.